The present invention relates to the measuring and monitoring of a stream or flow of gas; the flow being e.g. produced by a pump, for pumping a measuring gas through a measuring chamber of a gas analyzing instrument whereby in addition it is necessary to ascertain what interferences occur resulting from pressure and flow through variations as they may affect the measuring signal of that analyzer.
The gas flow in gas analyzers running and operating on a continuous basis, is usually monitored by means of devices that measure and indicate volume and/or mass flow and are in the following designated as flow quantity measuring devices. Particularly automatically operated gas analyzers but without continuous observation are advantageously equipped with and supplemented by a continuous flow measurement and by additionally monitoring limit values e.g. whether minimum flow conditions are no longer met, or maximum quantities of flow are exceeded. In other words it is monitored whether or not the gas flow remains between minimum and maximum values whereby of course in cases the limit is exceeded in one way or another certain warning signals issue permitting the taking of requisite steps to remedy the situation.
Certain gas analyzers react to variations in pressure in the measuring chamber on the basis of the specific physical principle involved in the measurement. Such pressure variations may cause an undesired modification of the measuring signals. Therefore in order to maintain the requisite accuracy of measurement it is either necessary to provide an absolute constancy of the pressure in the measuring chamber or to provide for an automatic correction that takes care of ensuing and existing pressure variations.
Simple gas measuring devices include mechanical, pneumatic measuring principles and simply measure the gas flow and monitoring the same. Here are used e.g. floating devices within a conically calibrated glass tube. The hovering height of the floating body such as a sphere indicates the amount of material that flows through, using a suitable calibration for the indication. The desired monitoring and signaling in case of a deviation is carried out on the basis of detection of maximum and minimum values. They are ascertained in that inductive, or optical magnetic transducer structures are arranged around this glass tube; when the floating body has reached a certain limit in one way or another (or both) interaction(s) result(s) in the production of a warning signal. Devices of this kind are disclosed in German printed patent application 25 04 275 and 14 98 896. Application 2,504,275 is based on an U.S. application, Ser. No. 441,997, filed 02/13/1974 and now U.S. Pat. No. 3,894,433.
Another way of monitoring limit values uses pneumatic electrical pressure sensitive switches responding to changes in static pressure of the gas analyzer. A device of this nature is for example disclosed in German printed patent application 12 11 423. Other pneumatic measuring systems use a measuring diaphragm and capillary device and respond to a differential pressure; arrangements of this kind are disclosed in German printed patent applications 15 23 019 and 19 03 353. A gas flow can also be ascertained by using in calorimetric systems and here one uses electrically heated wires which are (more or less) cooled by the (variable) gas flow. The cooling of the wire determines its resistance and any changes are ascertained by electric signal processing. Equipment of the kind is shown in German printed application 21 04 894.
In all these various methods when using an electrical output signal one uses a normal or physical indication of the flow through as well as limit monitoring under utilization of comparing adjustable desired or reference values with the actual values. Piezoelectric transducers are used often for measuring the pressure in a chamber, and the output of the transducer is used for correcting the pressure dependent influence on the measuring signal. The measuring signal output of such a transducer is thus introduced into an electrical signal processing circuit as generally pertaining to the analyzer.
The methods and equipments as described are using always at least two measuring transducers for obtaining and providing for the following three functions (i) measuring the quantity of flow through and indication of the rate of flow value; (ii) monitoring the limits of the flow through amount; and (iii) measuring the pressure in the measuring chamber for correcting the pressure dependent analyzing signal. Quite clearly this requires relatively extensive technical expenditure and is on one hand expensive and on the other hand prone to errors in its own right owing to the result in complexity.